It is known to transmit a signal, in particular a digital television or radio broadcast signal, via satellites or broadband transmission links, time-compressed for example by a factor of 20 and with a correspondingly increased frequency spectrum, for better utilization of the satellites and transmission links. In the case of television transmission, there would then be required for a feature film lasting 100 minutes a time period of only about 5 minutes. At the location where such a signal is received it is desired to record the signal, received in time-compressed form, especially as the said transmission often takes place during the night. Recording of such a compressed, broadband signal is not readily possible with conventional recorders. In addition, it is necessary to convert the signal transmitted in time-compressed form back into real time for reproduction via a customary receiver, that is to expand it again to the original time duration and the frequency spectrum of a standardized signal.
The object of the invention is to provide for such a transmission system with time-compressed transmission a process for recording and converting to real time which requires as few modifications as possible in a customary recorder.
In the solution according to the invention, the signal transmitted in time-compressed form is thus initially recorded without changing the time base by a recorder with helical-scan recording at a tape longitudinal speed increased by a factor with respect to the nominal tape longitudinal speed and at an increased head drum speed of rotation. The increase in the tape longitudinal speed and head drum speed of rotation makes it possible to record the time-compressed signal of greatly increased bandwidth. The tape longitudinal speed has in this case, for example, 4 times the value of the nominal tape longitudinal speed. The reproduction takes place at a tape longitudinal speed reduced with respect to the recording by a factor of, for example, 20. For example, the tape longitudinal speed during reproduction is reduced to ⅕ of the nominal tape longitudinal speed. The said factor of 4 in the increase of the tape longitudinal speed during recording and the factor of 5 in the reduction of the tape longitudinal speed during reproduction then give the said factor of 20. Reproduction takes place, however, at a head drum speed of rotation which is unchanged or changed little with respect to recording. The complete signal is then expanded to real time, for example 5 minutes to 100 minutes. However, the transmitted video signal of a frame continues to have the reduced value caused by the time-compressed transmission, of for example 1 ms. This signal would thus not be suitable initially for reproduction in a television receiver. Since part of the signal is in each case scanned in the reduced time of, for example, 1 ms, but the magnetic tape runs at the reduced tape longitudinal speed, associated signal components are scanned one after the other with overseaming (FIG. 4). In a further step of the invention, this signal is then fed to an electronic memory, into which, for example, during the standard duration of a frame of 20 ms 20 signal sections (FIG. 4) are read. These signals are then read out from the memory with the nominal frame duration of, for example, 20 ms.
The modified video recorder consequently assumes the task of expanding the signal for a transmission overall to real time. The subsequent electronic memory meanwhile assumes the task of converting, for example, in each case the video signal of a frame back to the nominal frame duration of 20 ms. By this sharing of the tasks on the one hand by a modified video recorder and on the other hand by an electronic memory, an inexpensive solution is achieved, in which in particular only few modifications are necessary to the customary video recorder.
Altogether, the following advantages are obtained by the solution according to the invention. The components and modules located in the signal path, such as for example the heads, the head drum, the rotation transformers of the head drum and the signal amplifiers have to be designed only for a narrow frequency range, since the video recorder operates only in the high frequency range of the time-compressed signal. If, in contrast to the invention, the video recorder were also to carry out the conversion to real time and to the low frequency position, these components would have to be designed for the high frequency range of the time-compressed signal and for the much lower frequency range of the customary television signal and, as a result would be extremely complicated and expensive. It is therefore also not necessary to provide separate heads for recording and reproduction. The tape longitudinal speed in the recorder is admittedly different by the said factor of, for example, 20. However, this does not give rise to any appreciable extra expenditure, because even on customary video recorders the capstan drive covers tape longitudinal speeds which are different by such a factor for special functions such as searching or single-frame advancement.
In the recording of a digital signal with consecutive numbering of the individual frames, it can be ensured even without great expenditure that the fast reading in of the time-compressed signal from the recorder and the slow reading out of the time-expanded signal for reproduction take place correctly with regard to the successive frames. It is admittedly possible for the video signals of some frames supplied by the recorder not to be correct or to suffer from interference. Since, however, during reproduction the video signal of a particular frame is multiply scanned and fed to the memory, it is ensured that sufficiently time-compressed frames with the correct video signal are always available. It is in this case also possible to feed the scanned video signal to an evaluation circuit, which initially checks the successive video signals of various time intervals and feeds to the memory only the video signal or signals which are found to be satisfactory. In principle, for a particular frame, the electronic memory only requires a single one of the video signals scanned one after the other. The invention can be used for a video signal and for an audio signal and is suitable in particular for digital signals of this type.